Setting an example

At my school the kids have a Christmas celebration party in the 6th Form. The students go to a hotel or restaurant and have a sit down meal followed by a disco or similar. Oddly these events are always held on a Sunday night and it’s an unwritten rule that the kids needn’t come into school the next day.
I whole-heartedly do not agree with this. Many of the organisers argue that they would prefer it to be on a Saturday or Friday night but they cannot book a venue as it’s so expensive around Christmas. This year I was asked if I’d like to go, but I said I wouldn’t be able to. Not because I was busy but because I refuse to set them this example.
The students seem to drink no matter if they have turned 18 or not in front of staff and then they are let off the hook from school the next day. Is that the kind of behaviour we should be expecting from young adults? Perhaps without supervision, yes. But to actively encourage students to participate in such events, I feel, is unprofessional and wrong. Surely we should just want kids to have fun but respect their responsibilities at the same time?